Hashimoto et al U.S. Pat. No. 3,856,588 discloses a process of introducing zinc as a dopant into a III-V compound substrate. A solution is prepared by dissolving a silicon acetate compound in ethanol containing a zinc compound as an impurity. The solution is spin coated on the surface of an N-type GaAs wafer and then heated at 250.degree. C. for 15 minutes to convert the silicon acetate to silica. The silica layer is then patterned and overcoated with a second silica layer differing from the first in omitting zinc. Alternatively, a silicon nitride layer can be formed in place of the second silica layer. Upon open tube heating to 850.degree. C. in a nitrogen atmosphere zinc is driven from the first layer into the III-V compound substrate to form a PN junction in the substrate.
The process of Hashimoto et al presents problems to those attempting a practical application of its teachings. In the course of introducing zinc as a dopant by the Hashimoto et al process the surface of the III-V compound substrate is degraded. When a polished wafer is employed as the III-V substrate, the surface of the wafer after zinc diffusion is uneven, appearing rough. A rough surface is, of course, undesirable, as it can lead to an uneven PN junction being formed in the wafer or in variations in PN junction depth from one area of the wafer to the next. Further, light emitting diodes fabricated by this process have exhibited high contact resistance when forward biased. This suggests that some type of resistive residue has been left on the surface of the substrate. Poor surface adhesion of the ohmic contact has also been observed.
A subsequent variation of the Hashimoto et al process has been to coat a silica layer lacking zinc before coating the silicon acetate layer containing zinc. Although this avoids substrate surface roughness, it creates its own disadvantage. The silica layer first put down on the substrate acts as a diffusion barrier for zinc. The means that any nonuniformity in the silica layer results in corresponding nonuniformities in the PN junction being formed, which in turn results in degraded diode performance.
Mir et al U.S. Pat. No. 4,880,770 discloses a process of preparing a superconductive thin film on a substrate. The superconductive thin film consists of rare earth alkaline earth copper oxide. The film is formed by coating each of the rare earth, alkaline earth and copper metals on the substrate in the form of metalorganic compounds, typically carboxylates. The metal carboxylates can be conveniently coated from a solution containing an organic solvent and a film-forming organic compound. Either or both of the ligands of the metalorganic compounds and the film-forming compound can contain organic moieties to promote film formation. The organic coating is heated to volatilize solvent and decompose the organic moieties remaining to form an intermediate layer. Subsequent heating to temperatures in the range of 900.degree. to 1100.degree. C. is employed to form a superconductive coating. While the substrate can in one form be a III-V compound wafer, it is specifically sought to avoid diffusion between the substrate and the superconductive layer as it is being formed. To this end a variety of barrier layers are suggested to avoid substrate-coating interactions.